High resiliency polyurethane foams are desirable for numerous applications, particularly as cushioning material for the seats and backs of upholstered furniture. The high resiliency and good load-bearing properties of these foams make it possible to eliminate the springs in upholstered furniture and car seats which were formerly required to achieve the desired load bearing properties using conventional foams. The properties of high resiliency foams resemble those of latex rubber and provide a high level of comfort.
The load bearing ability of a polyurethane foam is often determined using the Indentation Load Deflection (ILD) test (ASTM Test Method D-2406 Method A). The "comfort index" is defined as the ratio of the ILD value measured at 65% deflection to the value measured at 25% deflection. A ratio of 2.2 or greater is desirably for optimum seating comfort. Conventional cellular polyurethanes usually exhibit ratios of between 1.8 and 2.2. The procedure employed to measure indentation load deflection is described in a subsequent portion of this specification.
To achieve maximum seating comfort the weight or loading required for deflections up to 25% should be relatively low, thereby providing an initial sensation of softness. As the deflection passes the 25% level the loading required to obtain additional deflection should increase relatively rapidly in a non-linear manner to achieve the desired ultimate load bearing properties. The loading required to achieve 65% deflection should preferably be greater than one pound per square inch. In practical terms, if the foam is to be incorporated into a seat cushion, the ILD value at 65% deflection should be as high as possible, since this determines the minimum thickness of foam required to support a given weight. Even a small reduction in thickness can result in a considerable savings in material costs for a large scale operation.
High resiliency foams are conventionally prepared by reacting high molecular weight polyols (MW=4,500-6,000) which may contain grafted side chains of polyacrylonitrile or an acrylonitrile-styrene copolymer, with a polyfunctional isocyanate. At least a portion of the isocyanate component is usually polymeric, such as polymethylene polyphenyl isocyanate. Alternatively, a completely difunctional isocyanate, such as a mixture of 2,4- and 2,6-tolylene diisocyanates can be used in combination with a polyfunctional amine as a chain extending or crosslinking agent to maintain the crosslink density at the level achieved using a polymeric isocyanate. The reaction between polyol and isocyanate is usually catalyzed by a polymerization or "gel" catalyst. Organotin compounds, particularly derivatives of monocarboxylic acids such as dibutyltin dilaurate, are desirable gel catalysts because of the relatively short rise and "tack free" times that can be achieved. In addition, uncured foams are sufficiently coherent that they will not break apart when handled. This property is known as "green strength". The uncured foam can therefore be removed from the container in which it was formed. The decreased residence time is particularly desirable for a commercial operation, since it increases the output of the equipment employed to prepare the foam, measured on a pounds-per-hour basis.
As will be demonstrated in the accompanying examples, the highest ILD value that can be achieved at 65% deflection using conventional organotin gel catalysts is less than one pound per square inch. It is therefore an objective of this invention to define a class of gel catalysts which impart a higher load-bearing ability to polyurethane foams than can be achieved using these conventional organotin catalysts without sacrificing any of the desirable properties, such as rapid rise time and high green strength.
It has now been found that this objective can be achieved using certain organotin mercaptides or organotin derivatives of mercaptocarboxylic acid esters as the gel catalyst.